1. Field of the Invention
The present invention relates to a display panel formed by electrophotography, and to a method of manufacturing the same. In particular, the invention relates to a display panel including an image portion through which light emitted from a backlight is transmitted, and to a method for manufacturing the display panel. In the invention, the image portion refers to an image area formed with a color toner, and a background portion refers to an image area formed with a black toner.
2. Description of the Related Art
Conventionally, a backlighted display panel comprises a transparent plastic film having electrophotographically formed thereon a color image. The display panel is set on, for example, an overhead projector, which emits light from the backside of the display panel, whereby the emitted light is transmitted through the display panel to project the color image on a screen.
It is technically possible to further develop color image forming technology with respect to the plastic film used in the display panel. Namely, a new display panel comprises a plastic film having formed thereon an image portion and a background portion by fixing a color toner. The display panel is illuminated from its backside with a backlight, whereby light is transmitted only through the image portion of the display panel to display the image portion.
In contrast to display panels used for overhead projectors, display panels used for instrument panels of automobiles need to have image portions with excellent transparency and background portions with excellent light-sealing ability (i.e., an ability to prevent light from transmitted therethrough). While such transparency and light-sealing ability may be controlled in accordance with the concentration of a coloring agent in the color toner and toner mass per unit area on the plastic film, they are usually evaluated by transparent optical density, which is represented by a common logarithm (1/T) of a reciprocal of transmittance T of an illuminated light.
However, when many pinholes are present in the image portion and in the background portion, transmitted light leaks through the pinholes even if the image portion and the background portion satisfy a substantially preferable transparent optical density T across the entire surface. When such display panels are used for automobile instrument panels, they do not satisfy customers' design demands and are regarded as defective. The term “pinholes” used herein refers to tiny holes formed by toner not fixing to portions corresponding to the image portions and to the background portions on the plastic film.
As a result of detailed investigation into how pinholes are generated, the following was understood. In current electrophotography, since the toner is thermally fused at a temperature of about 150° C. or higher to fix the toner on the plastic film, air in the toner layer that forms the image portion and the background portion on the plastic film escapes from the toner layer during the fixing, whereby many pinholes having a circular diameter of about 0.2 mm or less form in the fixed image portion and background portion. When a display panel including an image portion and a background portion that contain many pinholes is illuminated from behind with a backlight, there are unsightly fatal defects in the image quality of the background portion in terms of light-sealing ability because the illuminated light is transmitted through the pinholes.